This invention is generally concerned with control methods, and is specifically concerned with a control method for use with a cryogenic refrigeration system of a type having a vapor powered motor for driving both an alternator for recharging the system battery, and a fan for blowing air through an evaporator-heater coil.
Air conditioning and refrigeration systems conventionally utilize a chlorofluorocarbon (CFC) refrigerant in a mechanical refrigeration cycle. Because of the suspected depleting effect of CFCs of stratospheric ozone (O.sub.3), practical alternatives to the use of CFCs in air conditioning and refrigeration systems are being sought. One such alternative is a cryogenic refrigeration system utilizing either liquid carbon dioxide or liquid nitrogen. Such a system is particularly attractive because, in addition to eliminating the need for CFC refrigerants, it also eliminates the need for a refrigerant compressor and the diesel engine or other prime mover that drives it.
An example of such a cryogenic refrigeration system is described and claimed in U.S. patent application Ser. No. 08/501,372, filed Jul. 12, 1995, and assigned to the Thermo King Corporation now U.S. Pat. No. 5,730,216 This particular system is preferably powered by liquid carbon dioxide, and includes an evaporator heater coil, an electronically controlled valve for modulating the amount of cryogenic gas that flows through the coil, and a vapor motor driven by the cryogenic gas that flows through the coil. The vapor motor is coupled to both an alternator for recharging the battery, and a fan for generating an air flow through the coil into a conditioned space. To allow the system to be operated in a heating mode, a vaporizer and superheater device is provided for heating the cryogenic gas to approximately 500.degree. F., as well as a set of solenoid operated valves for routing such superheated gas through the evaporator-heater coil in order to either defrost the coil, or heat the conditioned space.
For such a cryogenic refrigeration system to perform effectively, three basic criteria must be fulfilled. First, the system should rapidly achieve its temperature setpoint goal within a conditioned space with the expenditure of only a minimum amount of cryogen, since the amount of cryogen that can be carried in such a system is limited, while avoiding undesirable top-freezing of goods stored in the conditioned space. Secondly, the vapor motor of the system should be operated at sufficiently high speed to insure that the alternator coupled thereto effectively recharges the system battery, and the fan powered by the motor circulates a sufficient amount of air to avoid undesirable temperature nonuniformities throughout the conditioned space. Thirdly, the system should be operated in such a manner that temperature control of the conditioned space is arrive at quickly and smoothly, without subjecting the vapor motor to damage by flooding it with cryogen or subjecting it to solid CO.sub.2. This last criteria is particularly important, as cryogenic system are much more susceptible to damage as a result of an unwanted phase change of the refrigerant than mechanical refrigeration systems.
Clearly, there is a need for a method for controlling such a cryogenic refrigeration system so that only a minimum amount of cryogen is used in achieving the temperature set point within the conditioned space. It would further be desirable if such a method ran the cryogenically powered motor at speeds which were always sufficient for the alternator coupled thereto to adequately recharge the system battery, and the fan coupled thereto to circulate enough air in the conditioned space to avoid undesirable temperature non-uniformities therein. Such a control method should also be capable of modulating the amount of cryogenic gas entering the vaporizer and superheater device so that the gas exiting the vaporizer coil assembly was always above the freezing point of water, and the gas exiting the superheater gas assembly was approximately 500.degree. F., as the fulfillment of these criteria avoids the formation of unwanted water-ice on the vaporizer coil, and provides adequate heating without adverse metallurgical effects on the evaporator-heater coil. Finally, such a method should be capable of quickly and smoothly achieving a desired temperature set point without flooding the evaporator-heater coil or creating solid CO.sub.2 in the circuit, either condition of which could damage the vapor motor by subjecting it to liquid or solid cryogen.